An electric actuator for an electric brake system wherein the brake actuator is equipped with an electric motor of the above-mentioned type is disclosed, for example, in U.S. Pat. No. 5,219,049. The brake actuator includes an electric motor having a motor shaft to which rotation is imparted when the brake actuator is actuated. Brake linings are pressed against a brake drum or against a brake disc via a gear assembly. Furthermore, the brake actuator includes a locking brake which is built up as explained below. At the end of the motor shaft facing away from the gear assembly, a friction plate is mounted which rotates with the motor shaft when the electric motor is actuated. Opposite this friction plate, a further friction plate is mounted so that it cannot rotate and this friction plate is axially movable with respect to the first-mentioned friction plate. Usually, an air gap is present between the two friction plates so that the rotation of the motor shaft is unimpeded in both rotational directions when the brake actuator is actuated. However, if one wheel brake is to be locked with the aid of the brake actuator, then the brake linings are pressed against the brake drum by the actuation of the brake actuator and therefore a braking force is generated. Thereafter, and with the aid of a second electric motor, movement is imparted to the second friction plate in an axial direction so that the air gap between the two friction plates disappears and the two friction plates are pressed against each other so as to be friction tight. As a consequence of the foregoing, a rotation of the motor shaft of the first electric motor is prevented so that the brake actuator cannot be reversed by restoring forces and so that the previously adjusted braking force is maintained at the corresponding wheel.
The brake actuator disclosed in U.S. Pat. No. 5,219,049 is equipped with the locking brake and is so configured that the locking brake is without current in the "released state" as well as in the "unreleased state" and, in this way, the second motor, which is assigned to the locking brake, consumes a negligible current. The assembly of the electric locking brake is, however, complex. Furthermore, the brake actuator having a locking brake and disclosed in U.S. Pat. No. 5,219,049 exhibits an element of risk. More specifically, if the electric locking brake is unintentionally actuated because of a technical error or by the driver of the vehicle, this leads to a locking of the motor shaft of the electric motor of the brake actuator and the brake actuator can only then be actuated again when the electric locking brake is specifically released. An automatic release of the locking brake is not possible. In this way, a malfunction of the brake actuator occurs when there is an unintended and unplanned actuation of the locking brake.